The present invention relates to button air cells, and more particularly to the air electrode in such button air cells.
Miniature air cells, such as alkaline button air cells, are stable high-energy sources for electrical devices such as hearing aids. A continuous supply of air must be provided to the air electrode of the cell for the cell to work. Miniature alkaline air cells generally comprise an outer metal container having at least one air opening in its base to provide air to the active air electrode, a hydrophobic layer, an air electrode, a separator layer, an anode mass, and an electrolyte. The cell also generally includes a gasket and a metal cover to seal the open end of the cell, thereby sealing in the electrolyte.
Button air cells often use a polytetrafluoroethylene (PTFE) film as the hydrophobic layer. Other suitable materials are also known. The PTFE serves as an air transport layer and an electrolyte leakage barrier. The PTFE is normally laminated or bonded to the air electrode. However, heavy lamination of the PTFE to the air electrode shuts down the oxygen supply, causing mass transfer polarization, while low lamination may lead to delamination of the PTFE from the air electrode mix and premature failure of the cell.
Button air cells are used in a variety of different applications and thus are used in a variety of different climates. Humidity can affect the performance of the cell. Therefore, suitable bond strength and appropriate air and water permeability of the PTFE are essential for good cell performance. If the ambient humidity is high, the cell will gain water. This additional water takes up internal volume that is intended to accommodate cell discharge reaction products. As a result, internal pressure can build up in the cell, leading to cell bulging, leakage and reduced discharge capacity. The air electrode can also become flooded with electrolyte, leading to reduced discharge capacity. If the ambient humidity is low, the cell will loose water, which can cause deterioration of cell electrical characteristics and reduced discharge capacity.
U.S. Pat. No. 4,105,830 discloses an air depoloarized cell that compromises a laminated cathode assembly including an air cathode and an auxiliary cathode in combination with a layer of a thin nonporous gas permeable membrane, which is disposed with one side over the air cathode and with the opposite side having substantially unrestricted access to the atmosphere through a gas diffusion member. The nonporous membrane controls the transfer of oxygen from the ambient atmosphere to the air cathode exclusively by gas solubility with its permeability to oxygen selected to correspond to a predetermined average current density for the cell.
U.S. Pat. No. 5,587,259 discloses a metal current collecting substrate for an air cathode in an electromechanical metal air cell, wherein the substrate is hardened by one of the steps of sandblasting, shotblasting, plastic deformation of the substrate below the recrystallization temperature range of the metal thereof, and heating the substrate to above the transformation temperature of the metal, followed by quenching the substrate below the transformation temperature of the metal. Catalytically active materials are pressed or otherwise disposed upon the hardened substrate. The substrate is capable of being connected to electrical circuitry. The substrate can be a metal screen that has been hardened, roughened and pitted by sandblasting before the catalytically active materials are disposed thereupon, and before the substrate is incorporated into an electromechanical metal air cell.
In the past, two other approaches have also been taken to improve cells intended for use in high humidity environments. One is to reduce the electrolyte concentration so that the partial pressure of water inside the cell will be in equilibrium with the ambient water vapor pressure at a higher relative humidity. The other is to increase the void volume in the cell by reducing electrode and/or electrolyte volumes. Both of these approaches sacrifice discharge capacity in an effort to reduce cell bulging and leakage.
Thus, there is a need for an air electrode configuration for use in button cells that results in improved performance in high or low humidity conditions.
One aspect of the present invention is the process for making an air depolarized electrochemical cell comprising the steps of: (a) preparing an active layer of an air electrode comprising a catalytically active material and a binding material; (b) preparing a hydrophobic layer of an air electrode comprising a microporous membrane having an air permeability of about 50 to about 800 seconds; (c) laminating the active layer and the hydrophobic layer with sufficient force, applied uniformly across the entire surface of the electrode, to produce a laminated air electrode with an air permeability of about 5000 to about 20,000 seconds; and (d) combining the air electrode with a separator, a negative electrode, and an electrolyte in a.cell housing; wherein the air permeability is the time required for 0.153 in3 (2.5 cm3) of air under constant pressure of 12.2 inches of water (0.44 psi) to pass through a part of a sample 0.1 in2 (0.645 cm2) in area.
Another aspect of the present invention is an air depolarized electrochemical cell comprising a cell housing; a negative electrode; a laminated air electrode having an active layer comprising a catalytically active material and a binding material, a hydrophobic layer comprising a microporous membrane having an initial air permeability of about 50 to about 800 seconds prior to use within the cell, a laminated electrode air permeability of about 5000 to about 20,000 seconds, and a uniform density in the active area of the electrode; and a separator disposed between the negative electrode and air electrode.
Yet another aspect of the present invention is an air electrochemical cell comprising an air electrode having an air active material. The air electrochemical cell further comprises a polytetrafluoroethylene (PTFE) uniformly laminated on the air electrode wherein the PTFE-laminated air electrode has an air permeability from about 5000 seconds to about 20,000 seconds and an average peel strength of greater than about 90 g/in.